This invention relates to a method of improving dewatering efficiency, increasing sheet wet web strength, increasing sheet wet strength and enhancing filler retention in a papermaking process. Typically in a papermaking process chemicals are added in the wet end to assist in the dewatering of the slurry and improving wet or dry sheet strength. The wet end of the papermaking process refers to the stage in the papermaking process where the fiber is dispersed in the water in the slurry form. The fiber-water slurry then goes through drainage and dewatering process to form a wet web. The solid content after this wet formation process is about 50%. The wet web is further dried and forms a dry sheet of paper mat. Paper mat comprises water and solids and commonly 4 to 8% water. The solid portion of the paper mat includes fibers (typically cellulose based fibers) and can also include filler.
Fillers are mineral particles that are added to paper mat during the papermaking process to enhance the resulting paper's opacity and light reflecting properties. Some examples of fillers are described in U.S. Pat. No. 7,211,608. Fillers include inorganic and organic particles or pigments used to increase the opacity or brightness, or reduce the cost of the paper or paperboard sheet. Some examples of fillers include one or more of kaolin clay, talc, titanium dioxide, alumina trihydrate, barium sulfate, magnesium hydroxide, pigments such as calcium carbonate, and the like.
Calcium carbonate filler comes in two forms, GCC (ground calcium carbonate) and PCC (precipitated calcium carbonate). GCC is naturally occurring calcium carbonate rock and PCC is synthetically produced calcium carbonate. Because it has a greater specific surface area, PCC has greater light scattering abilities and provides better optical properties to the resulting paper. For the same reason however, PCC filled paper is weaker than GCC filled paper in dry strength, wet strength and wet web strength. Filler is generally much smaller than fiber, therefore, filler has much larger specific surface area than fiber. One of the challenges people found to increase filler content in the sheet is that high filler content decreases the efficiency of wet end chemicals, such as dewatering aids. This invention is to provide novel filler preflocculation, so that it reduced the adsorption of wet end chemicals onto filler surface, therefore, increased the efficiency of wet end chemicals such as dewatering aids.
Paper wet web strength is the tensile strength of a never dried sheet. Paper wet web strength is very critical for paper producers because increased paper wet web strength would increase machine runnability and reduce sheet breaks and machine down time. Paper wet web strength is a function of the number and the strength of the bonds formed between interweaved fibers of the paper mat. Filler particles with greater surface area are more likely to become engaged to those fibers and interfere with the number and strength of those bonds. Because of its greater surface area, PCC filler interferes with those bonds more than GCC.
Paper dewatering efficiency is also very critical for paper producers because decreased dewatering efficiency in wet end would increase steam demand for drying operation, reduce machine speed and production efficiency. Dewatering aids are widely used to improve dewatering efficiency for reducing energy consumption, increasing machine speed and production efficiency.
Paper wet strength is the tensile strength of the sheet when it is re-wet. Paper wet strength is not only one of important sheet properties, but also important for machine runnability for fine papermachine with a size press. Sheet gets re-wet after size press, and tends to break if the sheet wet web strength is low. Same as paper dry strength and wet web strength, paper wet strength decreases with the filler content in the sheet due to filler interference with fiber-fiber bonding.
Thus there is clear need and utility in methods and compositions for improving dewatering efficiency, increasing sheet wet web strength, increasing sheet wet strength and enhancing filler retention in a papermaking process. The art described in this section is not intended to constitute an admission that any patent, publication or other information referred to herein is “prior art” with respect to this invention, unless specifically designated as such. In addition, this section should not be construed to mean that a search has been made or that no other pertinent information as defined in 37 CFR §1.56(a) exists.